my red top interstate battery is 18 months old,always have battery tender on and the battery failed on me today.charged it and failed load test.will install new battery tomorrow also it is charging 13.8 volts with lights and stereo on.i have heard batteries in diablos last 2 years and is normal.
thanks

As far as i know it is good to change the battery after 18month or 24 months max even if you charge the battery .
The Battery cells just loose their ability to hold charge that is why it is good to change them before they fail.

You should get longer than this out of a battery. I had some batteries on a boat fail when nearly new, and when they were replaced under warranty I found out that maybe as many as 5% of new batteries are not perfect.

The second thing that could kill a battery this quickly is persistent overcharging, check what voltage the tender is floating. Also in the diablo it's not exactly the easiest place to check the battery electrolyte level, and if this is low the battery can die quickly.

I don't keep my car on a charger / tender any more because it was trying to maintain too high a voltage. I rely on the battery disconnect switch, and keep an eye on the actual voltage across the battery. Every once in a while will give it a boost charge to bring the battery up to 100% if the car is not doing many miles. [persistent undercharging and failure to recharge fully will also kill the battery]

hi bob i will only put the battery tender oncea week i was always keeping it on.